1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to the electrode structure of a semiconductor device having an irregular surface and a method for manufacturing the same.
2. Description of the Related Art
A semiconductor device is formed to have an irregular surface on the semiconductor substrate in many cases. For example, in a semiconductor laser, the electrode portion for supplying a current to a semiconductor active layer is formed in a stripe form on the active layer and the BH structure (Buried Hetero structure) is known as the electrode structure. The semiconductor laser of the above structure sometimes utilizes a structure in which groove portions are formed on both sides of the stripe-form active layer to cut apart the current block layers formed on both sides of the active layer. With the above structure, a bonding pad portion connected to the anode is formed in addition to the stripe-form electrode portion and the bonding pad portion is connected to the electrode portion via a wiring film formed in the groove portion. Thus, the electrode portion of the semiconductor substrate is formed on the flat surface portion of the complicated structure having an irregular surface. Therefore, the electrode forming method makes it necessary to utilize complicated steps.
Next, the semiconductor laser with the conventional structure having the groove portion is explained with reference to FIGS. 1, 2 and 3A to 3E. FIGS. 1 and 2 are schematic perspective views of an InGaAsP/InP-series semiconductor laser for optical communication, and FIGS. 3A to 3E are cross sectional views showing the steps of the semiconductor laser manufacturing process in due order.
As shown in FIG. 1, a plurality of light emission areas/waveguides (active layers) 110 of stripe form are formed on an n-InP semiconductor substrate 101 by the crystal growth. As the growth method, the LPE method (Liquid Crystal Epitaxy), MOCVD method (Metal-Organic Chemical Vapor Deposition) or the like is used and the above method is also used to form the following compound semiconductor layer. The active layer 110 is formed of undoped InGaAsP-series compound semiconductor. On both sides of the active layer 110, n-type InP layers (not shown) acting as current block layers and upper and lower p-type InP clad layers (not shown) disposed on the upper and lower surfaces of the current block layers are formed by the crystal growth. Part of the stacked body containing the current block layer is separated from the active layer 110 by means of stripe-form groove portions 105. The groove portions 105 are formed such that parts of the stacked body will be left behind on both sides of the active layer 110.
A p-type InGaAs contact layer (not shown) is formed on the upper p-type InP clad layer. The grooves are formed on both sides of the active layer 110 to construct a mesa structure. Since the groove portions 105 are formed such that parts of the stacked body will be left behind on both sides of the active layer 110, the mesa structure is formed with the active layer disposed between the parts of the stacked body. The surface of the semiconductor substrate 101 except the surface of the electrode portion is covered with an SiO.sub.2 insulative film 107. Further, stripe-form ohmic contact electrodes (AuZn) 112 for current injection are formed to cover the upper surfaces of the respective active layers 110.
As shown in FIG. 2, the ohmic contact electrode 112 is formed to cover the above structure and a lead-out electrode (Ti/Pt/Au) 113 for electrical connection is formed. Further, a bonding pad (Ti/Pt/Au) 114 is formed on the surface of the semiconductor substrate 101 by the vacuum deposition method. The lead-out electrode 113 and the bonding pad 114 are electrically connected to each other via a wiring formed on the internal portion of the groove portion 105 in the surface of the semiconductor substrate 101. Therefore, the lead-out electrode 113, bonding pad 114 and the wiring for electrically connecting them together are integrally formed and construct an element electrode as a whole.
Next, a method of manufacturing the above-described semiconductor laser device by use of the photolithography is explained with reference to FIGS. 3A to 3E. FIG. 3A is a cross sectional view taken along the 3A--3A line of FIG. 1. In FIG. 3A, the active layers, clad layers, current block layers and groove portions are already formed in the semiconductor substrate 101 and the stripe-form electrodes 112 for current injection are formed to cover the upper surface of the stripe-form active layers. An area of the main surface of the semiconductor substrate 101 except a portion in which the above electrode is formed is covered with an SiO.sub.2 insulative film 107 (FIG. 3A).
A metal film 116 formed of a stacked film of Ti/Pt/Au is uniformly formed on the entire portion of the main surface of the semiconductor substrate 101 by the vacuum deposition method. In this case, the electron beam vapor-deposition method of high directivity is used. An electrode material is deposited on the main surface of the semiconductor substrate 101 by use of an electron beam vapor-deposition system and the metal film 116 is deposited. Since the groove portions 105 are formed in the main surface of the semiconductor substrate 101 and stepped portions are formed, the electron beam vapor-deposition system having a rotation and revolution (planetary) mechanism is used to uniformly deposit the metal film also on the side portions of the stepped portions.
That is, the main surface of the semiconductor substrate which is a to-be-processed object is set to face the vapor-deposition source of the electron beam vapor-deposition system at an adequate tilt angle so that metal particles will be applied to the main surface of the substrate in a direction indicated by the arrow 115. Further, as shown in FIG. 3B, metal particles can be applied to and vapor-deposited on the main surface of the substrate in every directions by revolving the semiconductor substrate 101 in the B direction around a rotation axis (not shown) which is separated from and set in parallel to the rotation axis Y--Y passing substantially the center of the substrate and lies outside the substrate while rotating the semiconductor substrate 101 around the rotation axis Y--Y in the A direction. By the above method, the metal film 116 is uniformly deposited. Breakage of the metal film on the shoulder portion of the stepped portion can be prevented by using the electron beam vapor-deposition system having a rotation and revolution mechanism.
Next, a photoresist 117 is coated on the semiconductor substrate 101 to prevent breakage of the metal film on the shoulder portion of the stepped portion. Then, a photomask 118 is disposed on an area in which electrodes on the semiconductor substrate 101 are formed and ultraviolet rays 119 are applied to the photoresist 117 via the photomask to expose the photoresist and thus pattern the photoresist 117 (FIGS. 3C, 3D).
Next, the metal film 116 is etched with the patterned photoresist 117 used as a mask so as to form an electrode 120 formed of the stacked film of Ti/Pt/Au (FIG. 3E). The electrode is constructed by an electrode portion 113 for current injection, a bonding pad portion 114 for connecting a bonding wire for transferring a signal from the other circuit or semiconductor element to the semiconductor device, and a wiring portion for connecting the above portions together. Finally, the photoresist is removed to complete the semiconductor device as shown in FIG. 2. FIG. 3E is a cross sectional view taken along the 3E--3E line of FIG. 2.
In the above electrode forming method, the resist is formed in the electrode forming area after the metal film has been formed on the entire surface of the substrate and then an unnecessary portion of the metal film is removed by etching with the resist used as a mask. In the other electrode forming method, it is possible to vapor-deposit a metal film on the entire surface of the substrate by removing the resist on the electrode forming portion after the resist has been formed on the entire surface of the substrate, and applying metal particles in oblique directions while rotating and revolving the substrate. In this case, the unnecessary portion of the metal film is simultaneously removed by mechanically separating the resist after deposition of the metal film (lift-off method).
The thus constructed conventional semiconductor laser has the following problem in the structure and method.
1) In a case where the surface of the semiconductor substrate is made irregular, it is required to rotate and revolve the semiconductor substrate while it is held at an adequate tilt angle as described before in order to form the electrode along the irregular portion of the semiconductor substrate by the vapor-deposition method. To meet the requirement, a complicated special mechanism must be provided on the vacuum deposition device which is originally simple in construction.
2) Even if the vacuum deposition method of rotation and revolution type can be attained without using the special mechanism, the photolithography process effected for the substrate surface having a complicated irregular surface portion tends to cause a problem of breakage of a wire at the shoulder portion of the irregular surface portion of the photoresist and it is not a simple method.
3) Further, in a case where the vacuum deposition method of rotation and revolution type is effected, a difficult technique of forming the resist in a multilayered structure and making the resist thick is required in order to effect the lift-off method or the like.
The above problems 1) to 3) make it necessary to add the difficult technique to the photolithography process or vacuum deposition method which is originally a simple method. Addition of the difficult technique makes it difficult to manufacture the device in a simple construction on an experimental basis. Further, in the case of mass production, problems that the construction of the semiconductor manufacturing device is complicated, the number of process is increased, the yield is lowered and the like will occur.